Mass spectrometry (MS) is an analytical methodology used for quantitative and qualitative analysis of samples. Molecules in a sample are ionized and separated by a mass analyzer based on their respective mass-to-charge ratios. The separated analyte ions are then detected and a mass spectrum of the sample is produced. The mass spectrum may also provide information about the structural properties of the precursor masses by monitoring fragment species. In particular, mass spectrometry can be used to determine the molecular weights of molecules and molecular fragments within an analyte. Additionally, mass spectrometry can identify components within the analyte based on a fragmentation pattern.
Analyte ions for analysis by mass spectrometry may be produced by any of a variety of ionization systems. For example, Electrospray Ionization (ESI), Matrix Assisted Laser Desorption Ionization (MALDI) performed at high vacuum, intermediate or atmospheric pressure, Atmospheric Pressure Photoionization (APPI), Atmospheric Pressure Chemical Ionization (APCI) and Inductively Coupled Plasma (ICP) systems may be employed to produce ions in a mass spectrometry system. Many of these ionization sources generate ions at or near atmospheric pressure (760 Torr).
Once generated, the analyte ions are subsequently introduced into a mass spectrometer. Typically, the analyzer section of a mass spectrometer is maintained at high vacuum levels from 10−4 mbar to 10−8 mbar. In practice, sampling the ions includes transporting the analyte ions in the form of a narrowly confined ion beam from the ion source to the high vacuum mass analyzer chamber by way of one or more intermediate vacuum chambers.
Each of the intermediate vacuum chambers is preferably but not exclusively maintained at a vacuum level between that of the proceeding and following chambers. Therefore, the ion beam transports the analyte ions to progressively lower pressure vacuum regions in a stepwise manner from the pressure levels associated with ion formation to those of the mass analyzer. It is desirable to transportions through each of the various chambers of a mass spectrometer system with minimum ion losses. Often a Radio-Frequency (RF) ion guide is used to move ions in a defined direction to in the MS system.
Ion guides typically utilize RF electric fields to confine the ions radially while allowing or promoting ion transport axially. One type of ion guide generates a multipole field by application of a time-dependent voltage, which is often in the RF spectrum. These so-called RF multipole ion guides have found a variety of applications in transferring ions between parts of MS systems, as well as components of ion traps. When operated in presence of a buffer gas, RF guides are capable of reducing the velocity of ions in both axial and radial directions. This reduction in ion velocity in the axial and radial directions is known as “kinetic energy thermalization” or “translational cooling” of the ions via multiple collisions with neutral molecules of the buffer gas. Kinetically thermalized beams that are compressed in the radial direction are useful in improving ion transmission through narrow orifices of the MS system and reducing radial velocity spread in time-of-flight (TOF) instruments.
For purposes of clarity and consistency, the expression “field order of a multipole ion guide” as conventionally used in the art is meant to specifically refer to the number of electrical RF field poles produced by an equal-in-number corresponding elongated rods of an ion guide. Where the ion guide consists of multiple longitudinally traversing segments, the same expression may be used to refer to the number of electrical RF field poles produced by an equal-in-number corresponding elongated rods of a specific segment or sets of segments.
To give an example, a single segment quadrupole RF ion guide may be an ion guide (or section thereof) comprised four elongated rods. This type of multipole ion guide—when an appropriate RF potential is applied in a known sequence—is capable of producing four (4) RF field poles. The field order of the ion guide is thus commonly referred to as “order four” or “fourth order”. This is because the four (4) multipolar RF field poles define an “order four” (or “fourth order”), or simply, a quadrupolar electrical RF field distribution (or “quadrupolar field distribution” for short).
A multipole ion guide forming an RF electric field distribution operates to electrically influence the trajectory of ions traveling along a longitudinal axis. An ion guide operated as a collision cell may influence ion trajectories in both radial and axial dimension due to kinetic energy transfer induced by collisions with buffer gas molecules. By influencing the trajectory, it is commonly understood to imply that the ion trajectory or path is radially compressed toward the longitudinal axis. In cases where pressure is significantly low and so the ion mean free path is greater compared to the length of the system, the multipole ion guide simply acts as a transfer device to subsequent vacuum regions or compartments.
The longitudinal axis is the axis defined by the four elongated rods. As previously explained, the rods are typically cylindrically arranged about this longitudinal axis in any given segment. As for later segments in a multi-segment arrangement, each segment is likewise comprised of a separate set of elongated rods coupled structurally, electrically, or both, to the preceding stage.
The segments of a multi-segment ion guide all cooperate to structurally define a cylindrical or cylindrical-like ion guide. This ion guide includes an entrance end and an exit end with a common axis typically shared by all the segments which extends from the entrance end to the exit end.
In a related configuration, an octapole ion guide is a guide formed or defined by eight elongated rods to produce an “octapolar” electrical RF field distribution, or simply “octapolar field distribution.”
High-order field distributions are quite suitable in accepting ions characterized by extended kinetic energy and spatial spreads. Hence, from a kinetic energy and spatial spread perspective, the higher the field order distribution (meaning the greater the number of elongated rods), the greater the ion transfer efficiency. Unfortunately, high-order field distributions pose a challenge in the ability to radially compress an ion beam when travelling through an ion guide with a given narrow desired aperture disposed at the exit end. In contrast, lower order multipole ion guides exhibit a higher degree of ion radial compression at the expense, however, of the energy and spatial spreads they can tolerate.
One approach that has been proposed to deal with the challenge is to employ converging multipole rod geometries, as shown and described in U.S. Pat. No. 8,193,489. The convergence approach achieves wide kinetic energy and spatial spread acceptance in the region near or about the entrance end of the ion guide and enhanced focusing at the region near the exit end. This design approach does not provide a uniform field order distribution. If a uniform field order distribution is desirable, this approach may not be suitable. More important, the patented approach is limited by its very structure. It is not possible to, for example, provide commercially an ion guide with 24 poles (rods) which may be configurable in one or more mass spectrometers to achieve a wide range of field order distributions. This is a significant drawback in design.
Another approach is to provide a multi-segment ion guide with segments downstream having fewer elongated rods than those in segments upstream. This way, upstream segments which will be configured with higher number of rods than those downstream, and thus produce higher order field distributions, and vice versa.
Unfortunately, changing the number of rods from segment to segment introduces both design and cost challenges, and also fails to provide a suitable desired uniform field distribution. Transitions from higher order field distributions to lower order field distributions configured by a fewer number of rods are associated with fringe fields that can distort ion motion and produce significant ion losses.
What is needed, therefore, is an apparatus, which guides ions through a mass spectrometry system and that overcomes at least the shortcomings of known apparatuses.